Several clinicopathologic studies of primary mediastinal large B-cell lymphoma (Med-DLBCL) have been reported from Western countries; however, only a few series of at most 10 cases are available in Japan. To further clarify the Med-DLBCL occurring in Japan, we analyzed the clinical features of 28 patients with Med-DLBCL diagnoses who were treated at the National Cancer Center Hospital between 1982 and 2002. The median age was 37 years (range, 18-80 years). The ages of 16 male patients ranged widely from 18 to 80 years, whereas the 12 female patients appeared to show a single age peak at 20 to 40 years. Only 13 patients (46%) achieved a complete response with initial treatments, mostly by CHOP-like regimens (cyclophosphamide, doxorubicin, vincristine [Oncovin], and prednisolone) followed by radiotherapy. The estimated 3-year overall and failure-free survival rates were 32% and 33%, respectively, indicating the relatively unfavorable prognosis of the patients in our series. The following factors were found to be significantly associated with shortened survival prospects: age >60 years, serum lactate dehydrogenase level greater than normal, performance status >1, and presence of bulky mediastinal mass. In conclusion, the clinical features of Japanese patients with Med-DLBCL may be different from those with the disease in Western countries. Because this investigation was a single-institution study with a limited number of patients, however, multicenter confirmatory studies are needed.